Volume 38, Issue 11, November 2011
Index of content:
38(2011); http://dx.doi.org/10.1118/1.3653297View Description Hide Description
38(2011); http://dx.doi.org/10.1118/1.3605472View Description Hide Description
- MEDICAL PHYSICS LETTERS
K-space reconstruction with anisotropic kernel support (KARAOKE) for ultrafast partially parallel imaging38(2011); http://dx.doi.org/10.1118/1.3651693View Description Hide DescriptionPurpose:
Partially parallel imaging (PPI) greatly accelerates MRimaging by using surface coil arrays and under-sampling k-space. However, the reduction factor (R) in PPI is theoretically constrained by the number of coils (NC ). A symmetrically shaped kernel is typically used, but this often prevents even the theoretically possible R from being achieved. Here, the authors propose a kernel design method to accelerate PPI faster than R = NC .Methods:
K-space data demonstrates an anisotropic pattern that is correlated with the object itself and to the asymmetry of the coil sensitivity profile, which is caused by coil placement and B1 inhomogeneity. From spatial analysis theory, reconstruction of such pattern is best achieved by a signal-dependent anisotropic shape kernel. As a result, the authors propose the use of asymmetric kernels to improve k-space reconstruction. The authors fit a bivariate Gaussian function to the local signal magnitude of each coil, then threshold this function to extract the kernel elements. A perceptual difference model (Case-PDM) was employed to quantitatively evaluate image quality.Results:
A MR phantom experiment showed that k-space anisotropy increased as a function of magnetic field strength. The authors tested a K-spAce Reconstruction with AnisOtropic KErnel support (“KARAOKE”) algorithm with both MR phantom andin vivodata sets, and compared the reconstructions to those produced by GRAPPA, a popular PPI reconstruction method. By exploiting k-space anisotropy, KARAOKE was able to better preserve edges, which is particularly useful for cardiac imaging and motion correction, while GRAPPA failed at a high R near or exceeding NC . KARAOKE performed comparably to GRAPPA at low Rs.Conclusions:
As a rule of thumb, KARAOKE reconstruction should always be used for higher quality k-space reconstruction, particularly when PPI data is acquired at highRs and/or high field strength.
- RADIATION THERAPY PHYSICS
38(2011); http://dx.doi.org/10.1118/1.3641865View Description Hide DescriptionPurpose:
To quantify movement of prostate cancer patients undergoing treatment, using an in-house developed motion sensor in order to determine a relationship between patient movement and high dose rate (HDR) brachytherapy implant displacement.Methods:
An electronic motion sensor was developed based on a three axis accelerometer. HDR brachytherapy treatment for prostate is delivered at this institution in two fractions 24 h apart and 22 patients were monitored for movement over the interval between fractions. The motion sensors functioned as inclinometers, monitoring inclination of both thighs, and the inclination and roll of the abdomen. The implanted HDR brachytherapy catheter set was assessed for displacement relative to fiducial markers in the prostate. Angle measurements and angle differences over a 2 s time base were binned, and the standard deviations of the resulting frequency distributions used as a metric for patient motion in each monitored axis. These parameters were correlated to measured catheter displacement using regression modeling.Results:
The mean implant displacement was 12.6 mm in the caudal direction. A mean of 19.95 h data was recorded for the patient cohort. Patients generally moved through a limited range of angles with a mean of the exception of two patients who spent in excess of 2 h lying on their side. When tested for a relationship between movement in any of the four monitored axes and the implant displacement, none was significant.Conclusions:
It is not likely that patient movement influences HDR prostate implant displacement. There may be benefits to patient comfort if nursing protocols were relaxed to allow patients greater freedom to move while the implant isin situ.
On the role of the optimization algorithm of RapidArc® volumetric modulated arc therapy on plan quality and efficiency38(2011); http://dx.doi.org/10.1118/1.3641866View Description Hide DescriptionPurpose:
The RapidArc volumetric modulated arc therapy (VMAT) planning process is based on a core engine, the so-called progressive resolution optimizer (PRO). This is the optimization algorithm used to determine the combination of field shapes, segment weights (with dose rate and gantry speed variations), which best approximate the desired dose distribution in the inverse planning problem. A study was performed to assess the behavior of two versions of PRO. These two versions mostly differ in the way continuous variables describing the modulated arc are sampled into discrete control points, in the planning efficiency and in the presence of some new features. The analysis aimed to assess (i) plan quality, (ii) technical delivery aspects, (iii) agreement between delivery and calculations, and (iv) planning efficiency of the two versions.Methods:
RapidArc plans were generated for four groups of patients (five patients each): anal canal, advanced lung, head and neck, and multiple brain metastases and were designed to test different levels of planning complexity and anatomical features. Plans from optimization with PRO2 (first generation of RapidArc optimizer) were compared against PRO3 (second generation of the algorithm). Additional plans were optimized with PRO3 using new features: the jaw tracking, the intermediate dose and the air cavity correction options.Results:
Results showed that (i) plan quality was generally improved with PRO3 and, although not for all parameters, some of the scored indices showed a macroscopic improvement with PRO3. (ii) PRO3 optimization leads to simpler patterns of the dynamic parameters particularly for dose rate. (iii) No differences were observed between the two algorithms in terms of pretreatment quality assurance measurements and (iv) PRO3 optimization was generally faster, with a time reduction of a factor approximately 3.5 with respect to PRO2.Conclusions:
These results indicate that PRO3 is either clinically beneficial or neutral in terms of dosimetric quality while it showed significant advantages in speed and technical aspects.
Evaluation of brachytherapy lung implant dose distributions from photon-emitting sources due to tissue heterogeneities38(2011); http://dx.doi.org/10.1118/1.3641872View Description Hide DescriptionPurpose:
Photon-emitting brachytherapysources are used for permanent implantation to treatlungcancer. However, the current brachytherapydose calculation formalism assumes a homogeneous water medium without considering the influence of radiation scatter or tissue heterogeneities. The purpose of this study was to determine the dosimetric effects of tissue heterogeneities for permanent lungbrachytherapy.Methods:
The MCNP5 v1.40 radiation transport code was used for Monte Carlo(MC) simulations. Point sources with energies of 0.02, 0.03, 0.05, 0.1, 0.2, and 0.4 MeV were simulated to cover the range of pertinent brachytherapy energies and to glean dosimetric trends independent of specific radionuclide emissions. Source positions from postimplant CT scans of five patient implants were used for source coordinates, with dose normalized to 200 Gy at the center of each implant. With the presence of fibrosis (around the implant), cortical bone, lung, and healthy tissues,dose distributions andPTVDVH were calculated using the MCNP *FMESH4 tally and the NIST mass-energy absorption coefficients. This process was repeated upon replacing all tissues with water. For all photon energies, 109 histories were simulated to achieve statistical errors (k = 1) typically of 1%.Results:
The mean PTV doses calculated using tissue heterogeneities for all five patients changed (compared to dose to water) by only a few percent over the examined photon energy range, as did PTV dose at the implant center. ThePTVV100 values were 81.2%, 90.0% (as normalized), 94.3%, 93.9%, 92.7%, and 92.2% for 0.02, 0.03, 0.05, 0.1, 0.2, and 0.4 MeV sourcephotons, respectively. Relative to water, the maximum bone doses were higher by factors of 3.7, 5.1, 5.2, 2.4, 1.2, and 1.0 The maximum lungdoses were about 0.98, 0.94, 0.91, 0.94, 0.97, and 0.99. Relative to water, the maximum healthy tissuedoses at the mediastinal position were higher by factors of 9.8, 2.2, 1.3, 1.1, 1.1, and 1.1. However, the maximum doses to these healthy tissues were only 3.1, 7.2, 11.3, 10.9, 9.0, and 8.1 Gy while maximum bone doses were 66, 177, 236, 106, 49, and 39 Gy, respectively. Similarly, maximum lungdoses were 55, 66, 73, 74, 73, and 73 Gy, respectively.Conclusions:
The current brachytherapydose calculation formalism overestimates PTV dose and significantly underestimates doses to bone and healthy tissue. Further investigation using specific brachytherapysource models and patient-based CT datasets as MC input may indicate whether the observed trends can be generalized for low-energy lungbrachytherapydosimetry.
Evaluation of volumetric modulated arc therapy for cranial radiosurgery using multiple noncoplanar arcs38(2011); http://dx.doi.org/10.1118/1.3641874View Description Hide DescriptionPurpose:
To evaluate a commercial volumetric modulated arc therapy (VMAT), using multiple noncoplanar arcs, for linac-based cranial radiosurgery, as well as evaluate the combined accuracy of the VMAT dose calculations and delivery.Methods:
Twelve patients with cranial lesions of variable size (0.1–29 cc) and two multiple metastases patients were planned (Eclipse RapidArc AAA algorithm, v8.6.15) using VMAT (1–6 noncoplanar arcs), dynamic conformal arc (DCA, ∼4 arcs), and IMRT (nine static fields). All plans were evaluated according to a conformity index (CI), healthy braintissuedoses and volumes, and the dose to organs at risk. A 2D dose distribution was measured (Varian Novalis Tx, HD120 MLC, 1000 MU/min, 6 MV beam) for the ∼4 arc VMAT treatment plans using calibrated film dosimetry.Results:
The CI (0–1 best) average for all plans was best for ∼4 noncoplanar arc VMAT at 0.86 compared with ∼0.78 for IMRT and a single arc VMAT and 0.68 for DCA. The volumes of healthy brain receiving 50% of the prescribed target coverage dose or more (V 50%) were lowest for the four arc VMAT [RA(4)] and DCA plans. The average ratio of the V 50% for the other plans to the RA(4) V 50% were 1.9 for a single noncoplanar arc VMAT [RA(1nc)], 1.4 for single full coplanar arc VMAT [RA(1f)] and 1.3 for IMRT. The V 50% improved significantly for single isocenter multiple metastases plan when two noncoplanar VMAT arcs were added to a full single coplanar one. The maximum dose to 5 cc of the outer 1 cm rim of healthy brain which one may want to keep below nonconsequential doses of 300–400 cGy, was 2–3 times greater for IMRT, RA(1nc) and RA(1f) plans compared with the multiple noncoplanar arc DCA and RA(4) techniques. Organs at risk near (0–4 mm) to targets were best spared by (i) single noncoplanar arcs when the targets are lateral to the organ at risk and (ii) by skewed nonvertical planes of IMRT fields when the targets are not lateral to the organ at risk. The highest dose gradient observed between an organ at risk and a target at the edge of a VMAT arc plane or plane of IMRT fields was 17%/mm. The average absolute percent difference between the measured and calculated central axis dose for all the VMAT plans was 3.6 ± 2.2%. The measured perpendicular profile widths and shifts were on average within 0.5 mm of planned values. The average total MUs for VMAT plans was double the DCA average and similar to the IMRT average.Conclusions:
For the aforementioned planning and delivery system and cranial lesions greater than 7 mm in diameter, multiple noncoplanar arc VMAT consistently provides accurate and high quality cranial radiosurgerydose distributions with low doses to healthy braintissue and high dose conformity to the target. These qualities may make multiple noncoplanar arc VMAT suitable for a greater range of prescription doses or larger and more irregular lesions. For smaller and/or rounder lesions there are other clinically acceptable treatment techniques that may involve fewer couch angles or arcs and reduce treatment times.
A PENELOPE-based system for the automated Monte Carlo simulation of clinacs and voxelized geometries—application to far-from-axis fields38(2011); http://dx.doi.org/10.1118/1.3643029View Description Hide DescriptionPurpose:
Two new codes,PENEASY and PENEASYLINAC, which automate the Monte Carlo simulation of Varian Clinacs of the 600, 1800, 2100, and 2300 series, together with their electron applicators and multileaf collimators, are introduced. The challenging case of a relatively small and far-from-axis field has been studied with these tools.Methods:
PENEASY is a modular, general-purpose main program for the PENELOPEMonte Carlo system that includes various source models, tallies and variance-reduction techniques (VRT). The code includes a new geometry model that allows the superposition of voxels and objects limited by quadric surfaces. A variant of the VRT known as particle splitting, called fan splitting, is also introduced. PENEASYLINAC, in turn, automatically generates detailed geometry and configuration files to simulate linacs with PENEASY. These tools are applied to the generation of phase-space files, and of the corresponding absorbed dose distributions in water, for two 6 MV photon beams from a Varian Clinac 2100 C/D: a 40 × 40 cm2 centered field; and a 3 × 5 cm2 field centered at (4.5, −11.5) cm from the beam central axis. This latter configuration implies the largest possible over-traveling values of two of the jaws. Simulation results for the depth dose and lateral profiles at various depths are compared, by using the gamma index, with experimental values obtained with a PTW 31002 ionization chamber. The contribution of several VRTs to the computing speed of the more demanding off-axis case is analyzed.Results:
For the 40 × 40 cm2 field, the percentages γ1 and γ1.2 of voxels with gamma indices (using 0.2 cm and 2% criteria) larger than unity and larger than 1.2 are 0.2% and 0%, respectively. For the 3 × 5 cm2 field, γ1 = 0%. These figures indicate an excellent agreement between simulation and experiment. The dose distribution for the off-axis case with voxels of 2.5 × 2.5 × 2.5 mm3 and an average standard statistical uncertainty of 2% (1σ) is computed in 3.1 h on a single core of a 2.8 GHz Intel Core 2 Duo processor. This result is obtained with the optimal combination of the tested VRTs. In particular, fan splitting for the off-axis case accelerates execution by a factor of 240 with respect to standard particle splitting.Conclusions:
PENEASY and PENEASYLINAC can simulate the considered Varian Clinacs both in an accurate and efficient manner. Fan splitting is crucial to achieve simulation results for the off-axis field in an affordable amount of CPU time. Work to include Elekta linacs and to develop a graphical interface that will facilitate user input is underway.
38(2011); http://dx.doi.org/10.1118/1.3644842View Description Hide DescriptionPurpose:
The longitudinal dose ripple on the off-axis caused by helical radiation delivery, such as the TomoTherapy system, has been observed, and its relation with respect to pitch has been studied with empirically found optimal pitches, 0.86/n, by Kissick et al. [Med. Phys. 32, 1414–1423 (2005)]. This ripple artifact referred to as the thread effect is periodic in nature and is caused by various periodic factors. In this work, the factors that cause the thread effect were unveiled, including jaw profile divergence, the inverse square law, attenuation, and the cone effect, and their impact on the thread effect were studied.Methods:
Mathematical formulation for individual and combined factors were set up. Based on theoretical analysis and simulations, optimal pitches that result in local minima of the ripple amplitude with respect to the jaw width and off-axis distance were identified and verified. The effectiveness of optimization in reducing the thread effect were also studied.Results:
Analysis and simulation based on the square-shaped jaw profiles well characterize the thread effect. Simulations based on the real jaw profiles show reduced ripples and very good agreement of optimal pitches compared with those based on the square profiles. The optimal pitches were found to have little jaw width dependence, except for the real jaw profile of small width (1.05 cm). The optimal pitches for the real jaw profile of width 1.05 cm are unidentifiable except for the largest ones, due to the relative smoothness of the jaw profile. With optimized intensity modulation, the thread effect can be largely suppressed. For real jaw profiles, the optimal pitches with or without dose optimization do not change much. The numbers 0.86/n found by Kissick et al. well approximate the optimal pitches for off-axis distance of 5 cm. However, optimal pitches are not universal for different off-axis distances: they decrease as the off-axis distance increases.Conclusions:
The thread effect can be well explained by the proposed model. Optimization can largely reduce the thread effect. However, an optimal pitch reduces the ripple much easier especially when optimization is limited by many constraints. The optimal pitches predicted by the proposed model could be used as a reference for pitch selection regardless the tumor is at large or small off-axis distance.
38(2011); http://dx.doi.org/10.1118/1.3641645View Description Hide DescriptionPurpose:
In adaptive radiation therapy of prostate cancer, fast and accurate registration between the planning image and treatmentimages of the patient is of essential importance. With the authors’ recently developed deformable surface model, prostate boundaries in each treatmentimage can be rapidly segmented and their correspondences (or relative deformations) to the prostate boundaries in the planning image are also established automatically. However, the dense correspondences on the nonboundary regions, which are important especially for transforming the treatment plan designed in the planning image space to each treatmentimage space, are remained unresolved. This paper presents a novel approach to learn the statistical correlation between deformations of prostate boundary and nonboundary regions, for rapidly estimating deformations of the nonboundary regions when given the deformations of the prostate boundary at a new treatmentimage.Methods:
The main contributions of the proposed method lie in the following aspects.First, the statistical deformation correlation will be learned from both current patient and other training patients, and further updated adaptively during the radiotherapy. Specifically, in the initial treatment stage when the number of treatmentimages collected from the current patient is small, the statistical deformation correlation is mainly learned from other training patients. As more treatmentimages are collected from the current patient, the patient-specific information will play a more important role in learning patient-specific statistical deformation correlation to effectively reflect prostate deformation of the current patient during the treatment. Eventually, only the patient-specific statistical deformation correlation is used to estimate dense correspondences when a sufficient number of treatmentimages have been acquired from the current patient. Second, the statistical deformation correlation will be learned by using a multiple linear regression (MLR) model, i.e., ridge regression (RR) model, which has the best prediction accuracy than other MLR models such as canonical correlation analysis (CCA) and principal component regression (PCR).Results:
To demonstrate the performance of the proposed method, wefirst evaluate its registration accuracy by comparing the deformation field predicted by our method with the deformation field estimated by the thin plate spline (TPS) based correspondence interpolation method on 306 serial prostate CT images of 24 patients. The average predictive error on the voxels around 5 mm of prostate boundary is 0.38 mm for our method of RR-based correlation model. Also, the corresponding maximum error is 2.89 mm. We then compare the speed for deformation interpolation by different methods. When considering the larger region of interest (ROI) with the size of 512 × 512 × 61, our method takes 24.41 seconds to interpolate the dense deformation field while TPS method needs 6.7 minutes; when considering a small ROI (surrounding prostate) with size of 112 × 110 × 93, our method takes 1.80 seconds, while TPS method needs 25 seconds.Conclusions:
Experimental results show that the proposed method can achieve much faster registration speed yet with comparable registration accuracy, compared to the TPS-based correspondence (or deformation) interpolation approach.
38(2011); http://dx.doi.org/10.1118/1.3643028View Description Hide Description
In radiation therapy many motion management and alignment techniques rely on the accuracy of an internal fiducial acting as a surrogate for target motion within the lung. Although fiducials are routinely used as surrogates for tumor motion, the extent to which varying spatial locations in the lung move similarly to other locations has yet to be quantitatively analyzed. In an attempt to analyze the motion correlation throughout the lung, ten primary lungcancer patients underwent IRB-approved 4DCT scans in the supine position. Deformable registration produced motion vectors for each voxel between exhalation and inhalation. Modeling was performed for each vector and all surrounding vectors within the lung in order to determine the mean 3D Euclidean distance necessary for an implanted fiducial to correlate with surrounding tissue motion to within 3 mm (left lower: 1.7 cm, left upper: 2.1 cm, right lower 1.6 cm, and right upper 2.9 cm). No general implantation rule of where to position a fiducial with respect to the tumor was found as the motion is highly patient and lobe specific. Correlation maps are presented showcasing spatial anisotropy of the motion of tissue surrounding the tumor.
38(2011); http://dx.doi.org/10.1118/1.3651496View Description Hide DescriptionPurpose:
The aim of this study was to implement a protocol for reference dosimetry in tomotherapy and to validate the beam output measurements with an independent dosimetry system.Methods:
Beam output was measured at the reference depth of 10 cm in water for the following three cases: (1) a 5 × 10 cm2 static machine specific reference field (MSR), (2) a rotational 5 × 10 cm2 field without modulation and no tabletop in the beam, (3) a plan class specific reference (PCSR) field defined as a rotational homogeneous dose delivery to a cylindrical shaped target volume: plan with modulation and table-top movement. The formalism for reference dosimetry of small and nonstandard fields [Med.Phys.35: 5179–5186, 2008] and QA recommendations [Med.Phys.37: 4817–4853, 2010] were adopted in the dosemeasurement protocol. All ionization chambermeasurements were verified independently using alanine/EPR dosimetry. As a pilot study, the beam output was measured on tomotherapy Hi-art systems at three other centers and directly compared to the centers specifications and to alanine dosimetry.Results
: For the four centers, the mean static output at a depth of 10 cm in water and SAD = 85 cm, measured with an A1SL chamber following the TG-148 report was 6.238 Gy/min ± 0.058 (1 SD); the rotational output was 6.255 Gy/min ± 0.069 (1 SD). The dose stated by the center was found in good agreement with the measurements of the visiting team:D center/D visit = 1.000 ± 0.003 (1 SD). The A1SL chamber measurements were all in good agreement with Alanine/EPR dosimetry. Going from the static reference field to the rotational/non modulated field the dose rate remains constant within 0.2% except for one center where a deviation of 1.3% was detected.Conclusions:
Following the TG-148 report, beam output measurements in water at the reference depth using a local protocol, as developed at different centers, was verified. The measurements were found in good agreement with alanine/EPR dosimetry. The presented methodology may provide a good concept for reference dosimetry.
38(2011); http://dx.doi.org/10.1118/1.3651628View Description Hide DescriptionPurpose:
Leaf positions for dynamic multileaf collimator (DMLC) intensity modulated radiation therapy must be closely synchronized with MU delivery. For the Varian C3 series MLC controller, if the planned trajectory (leaf position vs. MU) requires velocities exceeding the capability of the MLC, the leaves fall behind the planned positions, causing the controller to momentarily hold the beam and thereby introduce dosimetric errors. We investigated the merits of a new commercial linear accelerator, TrueBeam™, that integrates MLC control with prospective dose rate modulation. If treatment is delivered at dose rates so high that leaves would fall behind, the controller reduces the dose rate such that harmony between MU and leaf position is preserved.Methods:
For three sets of DMLC leaf trajectories, point doses and two-dimensional dose distributions were measured in phantom using an ionization chamber and film, respectively. The first set, delivered using both a TrueBeam™ and a conventional C3 controller, comprised a single leaf bank closing at planned velocities of 2.4, 7.1, and 14 cm/s. The maximum achievable leaf velocity for both systems was 3 cm/s. The remaining two sets were derived from clinical fluence maps using a commercial treatment planning system for a range of planned dose rates and were delivered using TrueBeam™ set to the maximum dose rate, 600 MU/min. Generating trajectories using a planned dose rate that is lower than the deliverydose rate effectively increased the leaf velocity constraint used by the planning system for trajectory calculation. The second set of leaf trajectories was derived from two fluence maps containing regions of zero fluence obtained from representative beams of two different patient treatment plans. The third set was obtained from all nine fields of a head and neck treatment plan. For the head and neck plan, dose-volume histograms of the spinal cord and target for each planned dose rate were obtained.Results:
For the single closing leaf bank trajectories, the TrueBeam™ control system reduced the dose rate such that the leaf velocity was less than the maximum. Dose deviations relative to the 2.4 cm/s trajectory were less than 3%. For the conventional controller, the leaves repeatedly fell behind the planned positions until the beam hold threshold was reached, resulting in deviations of up to 19% relative to the 2.4 cm/s trajectory. For the two clinical fluence maps, reducing the planned dose rate reduced the dose in the zero fluence regions by 15% and 24% and increased the delivery time by 5 s and 14 s. No significant differences were noted in the high and intermediate dose regions measured using film. The DVHs for the head and neck plan showed a 10% reduction in cord dose for 20 MU/min relative to 600 MU/min sequencingdose rate, which was confirmed by measurement. No difference in target DVHs were observed. The reduction in cord dose increased total treatment time by 1.8 min.Conclusions:
Leaf sequencing algorithms for integrated control systems should be modified to reflect the reduced importance of maximum leaf velocity for accurate dosedelivery.
A comparison of postimplant dosimetry for 103Pd versus 131Cs seeds on a retrospective series of PBSI patients38(2011); http://dx.doi.org/10.1118/1.3651633View Description Hide DescriptionPurpose:
Permanent breast seed implantation (PBSI) is an accelerated partial breast irradiation technique performed using stranded103Pd radioactive seeds (average energy of 21 keV, 16.97 day half-life). Since 2004, 131Cs brachytherapy sources have become clinically available. The 131Cs radionuclide has a higher energy (average energy of 30 keV) and a shorter half-life (9.7 days) than 103Pd. The purpose of this study was to determine whether or not there are dosimetric benefits to using 131Cs brachytherapy seeds for PBSI.Methods:
The prescribed dose for PBSI using103Pd is 90 Gy, which was adjusted for 131Cs implants to account for the shorter half-life. A retrospective cohort of 30 patients, who have already undergone a 103Pd implant, was used for this study. The treatments were planned using the Variseed treatment planning system. The air kerma strength of the 131Cs seeds was adjusted in all preimplantation treatment plans so that the V100 (the volume within the target that receives 100% or more of the prescribed dose) were equivalent at time of implantation. Two month follow-up CT scans were available for all 30 patients and each patient was reevaluated using 131Cs seeds. The postimplant dosimetric parameters were compared using a two tailed t-test.Results:
The prescribed dose for131Cs was calculated to be 77 Gy; this dose would have the same biological effect as a PBSI implant with 103Pd of 90 Gy. The activities of the 131Cs sources were adjusted to an average of 2.2 ± 0.8 U for 131Cs compared to 2.5 ± 1.1 U for 103Pd in order to get an equivalent V100 as the 103Pd preimplants. While the use of 131Cs significantly reduces the preimplant V200 (the volume within the target that receives 200% or more of the prescribed dose) compared to 103Pd by 13.5 ± 9.0%, the reduction observed on the 2 month postimplant plan was 12.4 ± 5.1% which accounted for seed motion, implantation inaccuracies and tissue changes. This translates into an absolute reduction of 4.1 cm3 of tissue receiving 200% of the dose.Conclusions:
This analysis of 30 early stage breast cancer patients who underwent the PBSI procedure shows that there is a theoretical dosimetric advantage to using131Cs. However, in a realistic implant that will have seed misplacements and tissue changes, the use of 131Cs may not result in any clinically significant benefit.
38(2011); http://dx.doi.org/10.1118/1.3651695View Description Hide DescriptionPurpose:
The most common metric for comparing measured to calculated dose, such as for pretreatment quality assurance of intensity-modulated photon fields, is a pass rate (%) generated using percent difference (%Diff), distance-to-agreement (DTA), or some combination of the two (e.g., gamma evaluation). For many dosimeters, the grid of analyzed points corresponds to an array with a low areal density of point detectors. In these cases, the pass rates for any given comparison criteria are not absolute but exhibit statistical variability that is a function, in part, on the detector sampling geometry. In this work, the authors analyze the statistics of various methods commonly used to calculate pass rates and propose methods for establishing confidence intervals for pass rates obtained with low-density arrays.Methods:
Dose planes were acquired for 25 prostate and 79 head and neck intensity-modulated fields via diode array and electronic portal imaging device(EPID), and matching calculated dose planes were created via a commercial treatment planning system. Pass rates for each dose plane pair (both centered to the beam central axis) were calculated with several common comparison methods: %Diff/DTA composite analysis and gamma evaluation, using absolute dose comparison with both local and global normalization. Specialized software was designed to selectively sample the measuredEPID response (very high data density) down to discrete points to simulate low-density measurements. The software was used to realign the simulated detector grid at many simulated positions with respect to the beam central axis, thereby altering the low-density sampled grid. Simulations were repeated with 100 positional iterations using a 1 detector/cm2 uniform grid, a 2 detector/cm2 uniform grid, and similar random detector grids. For each simulation, %/DTA composite pass rates were calculated with various %Diff/DTA criteria and for both local and global %Diff normalization techniques.Results:
For the prostate and head/neck cases studied, the pass rates obtained with gamma analysis of high density dose planes were 2%–5% higher than respective %/DTA composite analysis on average (ranging as high as 11%), depending on tolerances and normalization. Meanwhile, the pass rates obtained via local normalization were 2%–12% lower than with global maximum normalization on average (ranging as high as 27%), depending on tolerances and calculation method. Repositioning of simulated low-density sampled grids leads to a distribution of possible pass rates for each measured/calculated dose plane pair. These distributions can be predicted using a binomial distribution in order to establish confidence intervals that depend largely on the sampling density and the observed pass rate (i.e., the degree of difference between measured and calculated dose). These results can be extended to apply to 3D arrays of detectors, as well.Conclusions:
Dose plane QA analysis can be greatly affected by choice of calculation metric and user-defined parameters, and so all pass rates should be reported with a complete description of calculation method. Pass rates for low-density arrays are subject to statistical uncertainty (vs. the high-density pass rate), but these sampling errors can be modeled using statistical confidence intervals derived from the sampled pass rate and detector density. Thus, pass rates for low-density array measurements should be accompanied by a confidence interval indicating the uncertainty of each pass rate.
38(2011); http://dx.doi.org/10.1118/1.3651698View Description Hide DescriptionPurpose:
Intensity modulated arc therapy (IMAT) is a radiation therapy delivery technique that combines the efficiency of arc based delivery with the dose painting capabilities of intensity modulated radiation therapy(IMRT). A key challenge in developing robust inverse planning solutions for IMAT is the need to account for the connectivity of the beam shapes as the gantry rotates from one beam angle to the next. To overcome this challenge, inverse planning solutions typically impose a leaf motion constraint that defines the maximum distance a multileaf collimator(MLC) leaf can travel between adjacent control points. The leaf motion constraint ensures the deliverability of the optimized plan, but it also impacts the plan quality, the delivery accuracy, and the delivery efficiency. In this work, the authors have studied leaf motion constraints in detail and have developed recommendations for optimizing the balance between plan quality and delivery efficiency.Methods:
Two steps were used to generate optimized IMAT treatment plans. The first was the direct machine parameter optimization (DMPO) inverse planning module in the Pinnacle3 planning system. Then, a home-grown arc sequencer was applied to convert the optimized intensity maps into deliverable IMAT arcs. IMAT leaf motion constraints were imposed using limits of between 1 and 30 mm/deg. Dose distributions were calculated using the convolution/superposition algorithm in the Pinnacle3 planning system. The IMAT plan dose calculation accuracy was examined using a finer sampling calculation and the quality assurance verification. All plans were delivered on an Elekta Synergy with an 80-leaf MLC and were verified using an IBA MatriXX 2D ion chamber array inserted in a MultiCube solid water phantom.Results:
The use of a more restrictive leaf motion constraint (less than 1–2 mm/deg) results in inferior plan quality. A less restrictive leaf motion constraint (greater than 5 mm/deg) results in improved plan quality but can lead to less accurate dose distribution as evidenced by increasing discrepancies between the planned and the delivereddoses. For example, the results from our patient-specific quality assurance measurements demonstrated that the average gamma analysis passing rate decreased from 98% to 80% when the allowable leaf motion increased from 3 to 20 mm/deg. Larger leaf motion constraints also led to longer treatment delivery times (2 to 4 folds) due to the additional MLC leaf motion.Conclusions:
Leaf motion constraints significantly impact IMAT plans in terms of plan quality, delivery accuracy, and delivery efficiency with the impact magnified for more complex cases. Our studies indicate that a leaf motion constraint of 2 to 3 mm/deg of gantry rotation can provide an optimal balance between plan quality, delivery accuracy, and efficiency.
Imaging of moving fiducial markers during radiotherapy using a fast, efficient active pixel sensor based EPID38(2011); http://dx.doi.org/10.1118/1.3651632View Description Hide DescriptionPurpose:
The purpose of this work was to investigate the use of an experimental complementary metal-oxide-semiconductor (CMOS) active pixel sensor (APS) for tracking of moving fiducial markers during radiotherapy.Methods:
The APS has an active area of 5.4 × 5.4 cm and maximum full frame read-out rate of 20 frame s−1, with the option to read out a region-of-interest (ROI) at an increased rate. It was coupled to a 4 mm thick ZnWO4 scintillator which provided a quantum efficiency (QE) of 8% for a 6 MV x-ray treatment beam. The APS was compared with a standard iViewGT flat panel amorphous Silicon(a-Si)electronic portal imaging device(EPID), with a QE of 0.34% and a frame-rate of 2.5 frame s−1. To investigate the ability of the two systems to image markers, four gold cylinders of length 8 mm and diameter 0.8, 1.2, 1.6, and 2 mm were placed on a motion-platform. Images of the stationary markers were acquired using the APS at a frame-rate of 20 frame s−1, and a dose-rate of 143 MU min−1 to avoid saturation. EPIDimages were acquired at the maximum frame-rate of 2.5 frame s−1, and a reduced dose-rate of 19 MU min−1 to provide a similar dose per frame to the APS. Signal-to-noise ratio (SNR) of the background signal and contrast-to-noise ratio (CNR) of the marker signal relative to the background were evaluated for both imagers at doses of 0.125 to 2 MU.Results:
Image quality and marker visibility was found to be greater in the APS with SNR ∼5 times greater than in the EPID and CNR up to an order of magnitude greater for all four markers. To investigate the ability to image and track moving markers the motion-platform was moved to simulate a breathing cycle with period 6 s, amplitude 20 mm and maximum speed 13.2 mm s−1. At the minimum integration time of 50 ms a tracking algorithm applied to the APS data found all four markers with a success rate of ≥92% and positional error ≤90 μm. At an integration time of 400 ms the smallest marker became difficult to detect when moving. The detection of moving markers using the a-SiEPID was difficult even at the maximum dose-rate of 592 MU min−1 due to the lower QE and longer integration time of 400 ms.Conclusions:
This work demonstrates that a fast read-out, high QE APS may be useful in the tracking of moving fiducial markers during radiotherapy. Further study is required to investigate the tracking of markers moving in 3D in a treatment beam attenuated by moving patient anatomy. This will require a larger sensor with ROI read-out to maintain speed and a manageable data-rate.
Auto-segmentation of normal and target structures in head and neck CT images: A feature-driven model-based approach38(2011); http://dx.doi.org/10.1118/1.3654160View Description Hide DescriptionPurpose:
Intensity modulated radiation therapy(IMRT) allows greater control over dose distribution, which leads to a decrease in radiation related toxicity. IMRT, however, requires precise and accurate delineation of the organs at risk and target volumes. Manual delineation is tedious and suffers from both interobserver and intraobserver variability. State of the art auto-segmentation methods are either atlas-based, model-based or hybrid however, robust fully automated segmentation is often difficult due to the insufficient discriminative information provided by standard medical imaging modalities for certain tissue types. In this paper, the authors present a fully automated hybrid approach which combines deformable registration with the model-based approach to accurately segment normal and target tissues from head and neck CTimages.Methods:
The segmentation process starts by using an average atlas to reliably identify salient landmarks in the patient image. The relationship between these landmarks and the reference dataset serves to guide a deformable registration algorithm, which allows for a close initialization of a set of organ-specific deformable models in the patient image, ensuring their robust adaptation to the boundaries of the structures. Finally, the models are automatically fine adjusted by our boundary refinement approach which attempts to model the uncertainty in model adaptation using a probabilistic mask. This uncertainty is subsequently resolved by voxel classification based on local low-level organ-specific features.Results:
To quantitatively evaluate the method, they auto-segment several organs at risk and target tissues from 10 head and neck CTimages. They compare the segmentations to the manual delineations outlined by the expert. The evaluation is carried out by estimating two common quantitative measures on 10 datasets: volume overlap fraction or the Dice similarity coefficient (DSC), and a geometrical metric, the median symmetric Hausdorff distance (HD), which is evaluated slice-wise. They achieve an average overlap of 93% for the mandible, 91% for the brainstem, 83% for the parotids, 83% for the submandibular glands, and 74% for the lymph node levels.Conclusions:
Our automated segmentation framework is able to segment anatomy in the head and neck region with high accuracy within a clinically-acceptable segmentation time.
An approach for online evaluations of dose consequences caused by small rotational setup errors in intracranial stereotactic radiation therapy38(2011); http://dx.doi.org/10.1118/1.3656954View Description Hide DescriptionPurpose:
The purpose of this work is to investigate the impact of small rotational errors on the magnitudes and distributions of spatial dose variations for intracranial stereotactic radiotherapy (SRT) treatment setups, and to assess the feasibility of using the original dose map overlaid with rotated contours (ODMORC) method as a fast, online evaluation tool to estimate dose changes (using DVHs) to clinical target volumes (CTVs) and organs-at-risks (OARs) caused by small rotational setup errors.Methods:
Fifteen intracranial SRT cases treated with either three-dimensional conformal radiation therapy (3DCRT) or intensity-modulated radiation therapy(IMRT) techniques were chosen for the study. Selected cases have a variety of anatomical dimensions and pathologies. Angles of ±3° and ±5° in all directions were selected to simulate the rotational errors. Dose variations in different regions of the brain, CTVs, and OARs were evaluated to illustrate the various spatial effects of dose differences before and after rotations. DVHs accounting for rotations that were recomputed by the treatment planning system (TPS) and those generated by the ODMORC method were compared. A framework of a fast algorithm for multicontour rotation implemented by ODMORC is introduced as well.Results:
The average values of relative dose variations between original dose and recomputed dose accounting for rotations were greater than 4.0% and 10.0% in absolute mean and in standard deviation, respectively, at the skull and adjacent regions for all cases. They were less than 1.0% and 2.5% in absolute mean and in standard deviation, respectively, for dose points 3 mm away from the skull. The results indicated that spatial dose to any part of the brainorgans or tumors separated from the skull or head surface would be relatively stable before and after rotations. Statistical data of CTVs and OARs indicate the lens and cochleas have the large dose variations before and after rotations, whereas the remaining ROIs have insignificant dose differences. DVH comparisons suggest that the ODMORC method is able to estimate the DVH of CTVs fairly accurately (within 1.5% of relative dose differences for evaluation volumes). The results also show that most of the OARs including the brain stem, spinal cord, chiasm, hippocampuses, optic nerves, and retinas, which were relatively distal from the skull and surface, had good agreement (within 2.0% of relative dose differences for 0.1 cc of the volumes ) between the ODMORC method and the recomputation, whereas OARs more proximate to the bone-tissue interface or surface, such as the lenses and cochlea, had larger dose variations (greater than 5.0%) for some cases due to the incapability of the ODMORC to account for scatter contribution variations proximate to interfaces and intrinsic dose calculation uncertainties for ROIs with small volumes.Conclusions:
The ODMORC method can be implemented as an online evaluation system for rotation-induced dose changes of CTVs and most OARs and for other related dose consequence analyses.
Accuracy of Acuros XB and AAA dose calculation for small fields with reference to RapidArc® stereotactic treatments38(2011); http://dx.doi.org/10.1118/1.3654739View Description Hide DescriptionPurpose
: To assess the accuracy against measurements of two photondose calculation algorithms (Acuros XB and the Anisotropic Analytical algorithm AAA) for small fields usable in stereotactic treatments with particular focus on RapidArc®.Methods
: Acuros XB and AAA were configured for stereotactic use. Baseline accuracy was assessed on small jaw-collimated open fields for different values for the spot sizes parameter in the beam data: 0.0, 0.5, 1, and 2 mm. Data were calculated with a grid of 1 × 1 mm2. Investigated fields were: 3 × 3, 2 × 2, 1 × 1, and 0.8 × 0.8 cm2 with a 6 MV photon beam generated from a Clinac2100iX (Varian, Palo Alto, CA). Profiles, PDD, and output factors were measured in water with a PTW diamonddetector(detector size: 4 mm2, thickness 0.4 mm) and compared to calculations. Four RapidArc test plans were optimized, calculated and delivered with jaw settings J3 × 3, J2 × 2, and J1 × 1 cm2, the last was optimized twice to generate high (H) and low (L) modulation patterns. Each plan consisted of one partial arc (gantry 110° to 250°), and collimator 45°. Dose to isocenter was measured in a PTW Octavius phantom and compared to calculations. 2D measurements were performed by means of portal dosimetry with the GLAaS method developed at authors’ institute. Analysis was performed with gamma pass–fail test with 3% dose difference and 2 mm distance to agreement thresholds.Results
: Open square fields: penumbrae from open field profiles were in good agreement with diamond measurements for 1 mm spot size setting for Acuros XB, and between 0.5 and 1 mm for AAA. Maximum MU difference between calculations and measurements was 1.7% for Acuros XB (0.2% for fields greater than 1 × 1 cm2) with 0.5 or 1 mm spot size. Agreement for AAA was within 0.7% (2.8%) for 0.5 (1 mm) spot size. RapidArc plans: doses were evaluated in a 4 mm diameter structure at isocenter and computed values differed from measurements by 0.0, −0.2, 5.5, and −3.4% for Acuros XB calculations (1 mm spot size), and of −0.1, 0.3, 6.7, and −1.2% for AAA, respectively for J3 × 3, J2 × 2, J1 × 1H, J1 × 1L RapidArc plans. Gamma Agreement Index from 2D dose analysis was higher than 95% for J3 × 3 and J2 × 2 plans, being around 80% for J1 × 1 maps. Sensitivity with respect to the dosimetric leaf gap and transmission factor MLC parameters was evaluated in the four RapidArc plans, showing the need to properly set the dosimetric leaf gap for accurate calculations.Conclusions
: Acuros XB and AAA showed acceptable characteristics for stereotactic small fields if adequate tuning of configuration parameters is performed. Dose calculated for RapidArc stereotactic plans showed an acceptable agreement against point and 2D measurements. Both algorithms can therefore be considered safely applicable to stereotactic treatments.